Heat oil in soup pot and sauté onion and garlic until soft and fragrant. Stir in tomato paste, cumin, pepper and chili powder. Add stock, lentils and carrots and simmer partially covered until lentils are tender, about 30 minutes. Blend half the soup in a blender and return to pot, or use an immersion blender. Add salt to taste and right before serving stir in lemon juice and cilantro. At the table, serve with a dollop of sour cream.